Future global positioning system (GPS) spacecraft employ three different L-band antennas including earth coverage (EC), military earth coverage (MEC) and regional military protection (RMP) antennas to broadcast the full set of GPS L-band signals. Each antenna may have a separate phase and group delay center, which can produce PNT errors if not suitably compensated. For example, during regular spacecraft yaw orbital maneuvers, required to reduce peak-to-peak thermal variations, MEC and RMP phase and group delay centers can rotate about the EC phase center creating an additional source of PNT errors for MEC and RMP users.
The existing GPS helix antenna arrays used for EC, MEC, and RMP transmit substantially high average continuous wave (CW) powers that are typically greater than 300 W CW per antenna. Thus, these tapered helix antennas are susceptible to high-power multipaction creating single-point failure risks. The EC antenna is particularly vulnerable to multipaction due to higher powers. The existing GPS EC antenna elements are interleaved with Ultra High Frequency (UHF) crosslink antennas and the MEC antenna elements are in close proximity to the UHF antenna. These close proximities increase the risk for passive intermodulation (PIM) in and near the antennas due to high signal strength from both UHF and L-band signals. Further, the existing GPS EC antenna L1 and/or L2 patterns have angular suck-outs (nulls) toward the space service volume (SSV) at about ±23.5° and/or ±26°, which diminishes transmitted signal power to geosynchronous satellite SSV users.